1. Field of the Invention
This invention relates to a catalyst for the polymerization of olefins and a process for preparing the same. More particularly, this invention relates to a catalyst for the polymerization of olefins which is capable of producing highly stereospecific olefin polymers, and a process for preparing the catalyst which is useful for the production of the above olefin polymers.
2. Description of the Prior Art
Hitherto, it was well-known that olefin polymers could be obtained in a high yield by polymerizing olefins using a ctalyst comprising a transition metal compound of the Groups IV to VI of the Periodic Table and a metal or an organometallic compound of the Groups I to III of the Periodic Table, i.e., a so-called Ziegler-Natta catalyst, and that the polymers produced by the above process would be generally in the form of a slurry of polymers where amorphous polymers are also produced as byproducts in addition to the crystalline olefin polymer.
In such a polymerization process, the bulk density of the olefin polymers produced generally affects the productivity of polymers and, therefore, and improvement in the bulk density is desirable in order to increase the efficiency of the reactor used for the polymerization.
Also, in olefin polymers, the stereospecificity of the polymers greatly affects the mechanical properties of the molded articles such as films, fibers and other articles prepared from the polymers.
Further, the production of amorphous polymers as by-products which are of commercially less value in the industrial utilization results in the loss of monomers used for the polymerization and, in addition, necessarily requires equipment for the removal of such amorphous polymers, thereby making the polymerization process uneconomical and disadvantageous from an industrial standpoint. Accordingly, it is considered that the polymerization process capable of producing polymers which are substantially free from amorphous polymers or which contain only a minimum amount of amorphous polymers is apparently one of the advantages in the production of polymers on an industrial scale.
On the other hand, the polymers obtained by such polymerization processes contain a residual catalyst which subsequently causes various problems such as instability and coloration of the olefin polymers, thus requiring equipment for the removal of the residual catalyst.
The above disadvantages associated with the conventional catalyst can be improved if the polymerization activity of the catalyst, i.e., a unit weight of the olefin polymer produced per a unit weight of the catalyst used for the olefin polymerization, can be increased, whereby the equipment required for the removal of the residual catalyst can be eliminated and the cost for the production of olefin polymers can be reduced.
A typical solid catalyst which has been conventionally used for the polymerization of olefins is titanium trichloride. The titanium trichloride catalyst is generally prepared from titanium tetrachloride by (1) reduction with hydrogen, (2) reduction with aluminum metal at high temperatures, (3) reduction with an organoaluminum compound at approximately room temperature, or the like.
Titanium trichloride obtained by the organoaluminum reduction of titanium tetrachloride has a .beta.-type crystal structure, and the polymer produced by polymerizing olefins using a catalyst comprising a combination of the above .beta.-type titanium trichloride and an organoaluminum compound has a practical problem because of its low stereospecificity, i.e., having a 70 to 80% content which is insoluble in boiling heptane.
The present inventors previously found that a solid catalyst which is useful for the production of olefin polymers having a high stereospecificity could be prepared by treating a .beta.-type titanium trichloride, which had been previously obtained by the reduction of titanium tetrachloride with an organoaluminum compound, with a certain type of complexing agent and subsequently treating the resulting catalyst with a monoalkyl aluminum dihalide. Although the above solid catalyst provides olefin polymers having remarkably increased stereospecificity and bulk density, it is still unsatisfactory in regard to catalyst activity during the polymerization and, therefore, is disadvantageous in that the residual amount of the catalyst after polymerization is not negligible.